Fracture toughening mechanisms acting in Mg-Y-Zn alloys with microstructural anisotropy and heterogeneity

A high-strength, fracture-resistant Mg 97 Y 2 Zn 1 alloy was developed by optimizing the microstructure and texture through conventional extrusion processing. This process produced a multimodal microstructure comprising dynamically recrystallized (DRXed) αMg phase grains and worked αMg and long-period stacking ordered (LPSO) phase grains. The volume fraction of worked αMg grains significantly increased by lowering the extrusion temperature and ram speed. The formation of worked αMg/LPSO grains with a fibrous texture, where <10 1 ̅ 0> is aligned along the extrusion direction, induced strong microstructural anisotropy in the extruded alloys. Tensile strength, fracture toughness, and crack-growth resistance in the longitudinal orientation improved with an increasing volume fraction and dispersion of worked αMg grains. During fracture toughness testing, macroscopic crack branching was observed in specimens with a high fraction of worked grains, which served as the primary toughening mechanism for this alloy. Additionally, shear ligament toughening further enhanced the crack propagation resistance. The basal planes in worked αMg/LPSO grains were oriented perpendicular to the original notch plane, enabling high-angle crack deflection and branching. Achieving a well-dispersed worked αMg/LPSO grain structure is essential for LPSO-type Mg alloys to attain high strength and fracture toughness. • A high-strength, fracture-resistant Mg 97 Y 2 Zn 1 alloy was developed by optimizing the microstructure and texture. • Tensile strength and fracture toughness improved with an increasing volume fraction and dispersion of worked αMg grains. • The simultaneous achievement of high strength and fracture resistance is facilitated by crack deflection and branching.